Timing recovery in communication systems is related to the process of identifying symbol boundaries in a received signal, so that each symbol can be windowed and processed separately so that its value can be determined. In some communication systems such as OFDM (other examples include IFDMA, Single Carrier with cyclic extension), a guard interval is inserted between successive symbols to overcome inter-symbol interference (ISI) caused by multipath delay-spread in the communication channel. Usually each symbol is cyclically extended with a prefix and/or a postfix to cover the guard interval. The cyclic extension absorbs the delay-spread and thus keeps the data portion of the symbol free of ISI. When the channel delay-spread is less than the duration of the cyclic extension, only a portion of the cyclic extension is corrupted while the rest remains ISI-free. This creates ambiguity in the timing recovery process because there is more than one possible position of the sampling-window for obtaining an ISI-free representation of the symbol.
Although known methods for timing recovery in OFDM systems are adequate and beneficial in many situations, they present several shortcomings. In a method proposed by Jan-Jaap van de Beek et al., “ML Estimation of Time and Frequency Offset In OFDM Systems”, IEEE Transactions on Signal Processing, vol. 45, no. 7, July 1997 (hereinafter “Jan-Jaap van de Beek”), the cyclic extension is used to identify the best sampling position. FIG. 1 is a conceptual illustration of the Jan-Jaap van de Beek method of timing recovery. As shown, the method combines consecutive correlation values together or the length of the cyclic extension.
Unfortunately, this method is optimized and well suited for a single-ray channel. As a consequence, in a multipath-fading channel, the sampling position picked by this method may not be free of ISI and it can waver depending on which of the multipath rays is the strongest at a particular time. In a multipath-fading channel, these shortcomings detract from one of the purposes behind OFDM, which is to reduce the effects of multipath fading.
In another method proposed by T. M. Schmidl et al., “Low-Overhead, Low-Complexity [Burst] Synchronization for OFDM”, Proceedings of ICC 1996, vol. 3, pp. 1301-1306 (hereinafter “Schmidl”), a special training symbol is used to estimate a sampling position. A disadvantage of this method is that the sampling position picked by this method can jump about within a set of valid positions, leading to jitter in the timing estimates. The jitter makes it difficult to use averaging, e.g., a phase locked loop (PLL), to obtain a steady sampling position. Another disadvantage with this method is that the timing estimate is based on the training symbol alone and there is no averaging over the fading process. This creates problems in a fast fading scenario where the instantaneous delay profile at the training symbol could be significantly different from the actual channel power delay profile.
Further, the above mentioned methods are not necessarily intended to provide an estimate of the delay-spread in the channel.
Accordingly, there is a need for an improved method of timing recovery and delay-spread estimation in communication systems employing cyclic extensions.